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The 2,400-year search for the atom - Theresa Doud

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How do we know what matter is made of? The quest for the atom has been a long one, beginning 2,400 years ago with the work of a Greek philosopher and later continued by a Quaker and a few Nobel Prize-winning scientists. Theresa Doud details the history of atomic theory.

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Ernest Rutherford

Although Ernest Rutherford is well known for his discovery of the nucleus, he did a lot of other research and experiments into topics other than the atom. He started his career studying electricity and magnetism and it wasn’t until he left his home of New Zealand and moved to Cambridge, England that he started working with the atom. Working under J.J. Thomson at Cambridge University, Rutherford investigated electrical conduction in gases using X-rays. When radioactive atoms were discovered in the late 1890s, he switched his experiment to use radioactivity instead of X-rays to initiate electrical conduction in his gases. While working with the newly discovered radioactive atoms, he lost interest in electrical conduction and switched to studying radioactivity. Soon he discovered that there were two forms of radioactivity being emitted from atoms and he named these emissions, alpha and beta. Around the turn of the century, Rutherford moved to Montreal, Canada to continue his research at McGill University and it was here while conducting experiments with thorium that he discovered radon, the radioactive noble gas (although he is not credited directly with the discovery). He published his first book called Radio-activity in 1904 over the decay of heavier radioactive atoms into lighter atoms. He won a Nobel Prize in 1908 ‘for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.’

In 1911 after returning to England, Rutherford conducted his most famous experiment with alpha particles and gold foil which lead to his discovery of the nucleus. Watch this video and learn more about Rutherford’s discovery. Toward the end of World War 1, after helping the British with antisubmarine research, Rutherford returned to his alpha particles and elements and became the first true “alchemist” when he changed nitrogen atoms into oxygen atoms by bombarding them with alpha particles. In 1919 he became the Director of Cambridge University’s Cavendish Laboratory, the very place he started his journey with the atom. After a knighthood and also an elevation into the British peerage to become Lord Rutherford, he continued his work with the atom helping with James Chadwick’s discovery of the neutron as well as with John Cockcroft and Ernest Walton’s research into artificially splitting the atom. He died in 1937 at the age of 66 due to a strangulated hernia. He is buried at Westminster Abbey near J.J. Thomson, Lord Kelvin and Sir Isaac Newton.

Where are the Electrons?

After Rutherford discovered the nucleus, Neils Bohr (another of J.J. Thomson’s students) improved upon his model by putting the electrons in specific energy levels around the nucleus. Today when most people are asked to draw an atom, they will simply draw a dot with circles around it. This is the Bohr model of the atom… and it is not the true model any longer. After Bohr, a couple of other scientists, namely Werner Heisenberg and Erwin Schrodinger, helped to answer the question: “Where are the electrons?”. Louis de Broglie proposed in 1924 that matter behaves as both particles and waves, called particle-wave duality. Schrodinger actually devised an equation for this phenomenon which is called the “wave equation.”

When Schrodinger’s equation is solved it does not give the exact location of an electron around a nucleus but as Max Born (yet another student of Thomson) discovered, it gives a probability density or area where the electron might be found. We call these probability densities, orbitals. There are four types of orbitals, each given a letter to signify their shape: s, p, d or f. Since according to Werner Heisenberg’s uncertainty principle, we can not know the location and also the velocity of an electron, these probability densities, or orbitals, are the best we have for describing an electron’s location at a certain time.

Schrodinger later went on to come up with a thought experiment or paradox that is referred to as Schrodinger’s Cat. This thought experiment gives an even more confounding view of quantum mechanics and how little we really know about it.

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